1. Field of the Invention
The present invention relates generally to control rod drives for nuclear reactors and particularly to mechanisms for preventing an undesired ejection of the leadscrew of the control rod drive.
2. Description of the Prior Art
Control rod drives for nuclear reactors comprise sealed pressure vessels which are sealably mounted to the nuclear reactor to connect the control rods of the reactor and to allow the controlled positioning of the rods within the reactor. As such the housing containing the drive mechanism is maintained at the same pressure as the nuclear reactor. Should the housing of the control rod drive rupture, the leadscrew of the drive mechanism along with the control rod connected thereto could be ejected from the housing by the pressure difference created between the housing and the ambient.
Known prior art control rod drives depend upon the engagement of the driving element with the leadscrew to prevent the leadscrew from being ejected. However, certain situations are encountered where it is desirable to "scram" or speedily insert the control rod into the nuclear reactor to shut down the reactor. Such a "scram" is usually provided by disengaging the driving element from the leadscrew and allowing the leadscrew and the attached control rod to drop into the reactor. Thus, during a "scram" condition the leadscrew and control rod could be most easily ejected if a rupture were to occur in the housing. A similar condition exists when the reactor is shut down or anytime that the drive is disengaged from the leadscrew. Even when engagement of the leadscrew with the drive is provided, should a rupture occur which would subject the leadscrew to the full pressure of the reactor the engagement between the leadscrew and the drive could be broken or stripped and ejection may still occur.